1. Field of the Invention
The field of the present invention is the purification of drinking water by removal of biocolloids including protozoan parasites.
2. The Prior Art
Cryptosporidium parvum is a protozoan parasite commonly found in surface waters contaminated with sewage and animal wastes, such as streams, lakes and rivers. The life cycle of Cryptosporidium includes the formation of oocysts, which are excreted in the stools of infected individuals. Infection occurs with the oral ingestion of oocysts in contaminated water. Oocysts have been shown to be highly environmentally resistant and are capable of withstanding various drinking water filtrations and chemical treatments such as chlorination. There have been six major outbreaks of cryptosporidiosis in the U.S. that have resulted from contaminated drinking water, all of which occurred in communities in compliance with all state and federal drinking water standards.
Existing drinking water regulations require public water systems that use surface water sources and provide filtration to achieve at least a 99 percent (2-log) removal of Cryptosporidium. These regulations are adequate for the majority of public water systems; however a subset of systems with higher vulnerability to Cryptosporidium exists, including those systems with the highest source-water Cryptosporidium levels. For those systems, additional treatment is necessary.
The removal of Cryptosporidium from drinking water is currently achieved through the use of filtration systems. Operational failure of these filtration units has been recognized as one of the causes responsible for outbreaks of Cryptosporidium parvum in water treatment systems. Suboptimal coagulation pretreatment has been found to decrease the Cryptosporidium removal by orders of magnitude. While conventional filtration can give a high degree of removal, there are a number of operational parameters that need to be optimized for successful operation of filtration systems, in particular, the coagulation system.
The conventional treatment for the removal of Cryptosporidium parvum oocysts generally consists of a series of processes including coagulation, flocculation, sedimentation and filtration. Direct filtration, which is typically used on sources with low particulates levels, includes coagulation and filtration, but not sedimentation. Other common filtration processes include slow sand, diatomaceous earth, membranes, and bag and cartridge filters. The effectiveness of conventional and direct filtration treatments has been shown to be dependent on the pretreatment effectiveness (i.e., optimal coagulation), as well as particle size and concentration. Conventional treatment practices are generally capable of meeting 2 to 3-log10 removals in most of the cases subject to optimal pretreatment (Huck, P. M. et al., “Effects of filter operation in Cryptosporidium research and research needs.” AWWA Research Foundation and American Water Works Association, Denver, Colo. 1995). Alternative technologies such as diatomaceous earth filtration (Ongerth, J. E and Pecorato, J. P. “Testing of diatomaceous earth filtration for removal of Cryptosporidium oocysts.” J. AWWA, 2001, vol. 93, pp. 54-63.), dissolved air flotation (Plummer, J. D. et al., “Removing Cryptosporidium parvum by dissolved air flotation.” J. AWWA, 1995, vol. 87, pp. 85-95.), and slow sand filtration (Fogel, D. et al., “Removing giardia and Cryptosporidium by slow sand filtration.” J. AWWA, 1993, vol. 88, pp. 77-84.) seem capable of achieving comparable, or even greater, levels of Cryptosporidium removal.; however, issues such as flow rate (diatomaceous earth filtration), filtrate turbidity (dissolved air flotation), and water temperature (slow sand filtration) have been shown to affect optimal performance of these methods. Of the methods available to the drinking water industry, membrane processes provide the most satisfactory removal of Cryptosporidium parvum; however these systems have been prohibitively expensive for large public supply systems (Frey, M. M. et al., “Critical evaluation of Cryptosporidium research and research needs.” AWWA Research Foundation and American Water Works Association, Denver, Colo. 1995). Highly variable treatment performance has been observed for bag and cartridge filtration systems, depending on the particular filter tested.